Semiconductor circuits are commonly powered by a variety of means. In some cases, the circuits are powered solely from an external source coupled to a power supply terminal. However, in other cases, one or more voltages having a magnitude and/or polarity that is different from the magnitude and polarity of a voltage supplied to the circuit may be needed. One common technique for providing such voltages is through use of an internal circuit known as a charge pump. An advantage of using a charge pump is that it may be configured to supply a voltage having a magnitude that is greater than the magnitude of an external supply voltage powering the charge pump. Furthermore, it may be configured to supply a voltage that alternatively or additionally has a polarity that is different from the polarity of the external supply voltage. However, one disadvantage of using a charge pump is that they often may have somewhat limited efficiency. As a result, power may be undesirably wasted in converting one voltage to another voltage having a different magnitude or polarity.
The relative inefficiency commonly encountered with charge pumps makes it all the more important to use the power generated by charge pumps as efficiently as possible. For example, using power in a circuit having an efficiency of only 80% may result in an effective efficiency of only 64% if the circuit is supplied with power by a charge pump also having an efficiency of 80%.
Power is commonly consumed in semiconductor circuits in a variety of situations. One situation that commonly consumes power is transitioning a signal line from one binary voltage to another. For example, signal lines are commonly driven by an inverter having two complementary transistors coupled in series between two supply voltages. An output signal line may then be coupled to the transistors at a node where they are coupled to each other. The line normally transitions from one voltage to another by turning OFF one of the transistors while the other transistor is being turned ON. During this transition, both transistors are often partially conductive at the same time so that current flows from one supply voltage to the other, thereby consuming significant power. The power consumption could be avoided by turning one transistor OFF before starting to turn the other transistor ON, but doing so would increase the time required to transition the signal line from one voltage to the other. Insofar as high switching speed may be very important, this power saving alternative may not be practical in many situations.
Another phenomena that commonly consumes power when transitioning of a signal line from one binary voltage to another results from the capacitive nature of many signal lines. Signal lines, particularly long signal lines, may have substantial capacitance, which allows them to store substantial charge. This charge should be dissipated in order to transition the signal line from one voltage level to another. For example, if the signal line is driven to a first supply voltage VCC, sufficient current should be provided to charge the signal line to that level. If the signal line is subsequently discharged to a second supply voltage, such as ground potential, the signal line may be discharged to that level. Thus, each charge and discharge cycle may effectively result in current flowing from VCC to ground, thereby consuming power.
Power may also be consumed in other ways by a wide variety of digital circuits. Yet, to the extent possible, it would be desirable to minimize power consumption in semiconductor circuits, particularly where the semiconductor circuits are powered by a charge pump.